Skeletal muscle growth is accompanied by the proliferation and fusion of satellite cells, a process which produces the required increase in postmitotic myonuclei. Satellite cells serve a dual function because in addition to providing a source of myonuclei they also provide a source of myoblasts for muscle regeneration. Our aim is to examine the ways in which satellite cell proliferation and fusion behavior is modified in vivo to accommodate the different requirements for myonuclear accretion in muscles that grow at markedly different rates and to obtain a better understanding of the inter-relationships of growth and regeneration potential in this cell population. Cell cycle times, labeling indices, and fusion indices of satellite cells will be measured in the rapidly growing soleus and slower growing EDL using 3H and/or 14C labeled thymidine as markers. Continuous labeling experiments using 3H thymidine will be carried out to determine if all of the satellite cell population is active in the production of myonuclei or whether a portion of the population is held in reserve for activation only in the event of muscle injury. A second aspect will examine the relationship between the number of satellite cells and the efficiency of muscle regeneration. Various lengths of the EDL muscle will be killed by a freezing and thawing procedure. The satellite cells in the frozen portions will also be killed so the muscle regeneration that follows is a product of only those cells remaining in the viable non-frozen portion. Since satellite cells are equally distributed along the length of myofibers, the number of cells remaining in the viable portion of the muscle can be estimated. The aim is to determine what critical mass of cells is required to regenerate a muscle. The ability of implanted cells to aid in the regeneration will also be examined.